1. Field of the Invention
The present invention relates to a thread tensioning device for a sewing machine which can provide effective tension timely to the thread during a thread loosening process to avoid the occurrence of skip stitch while effectively improving the sewing quality.
2. Description of the Prior Art
In order to provide a proper amount of tension to the thread while the thread hook loosens the thread, a sewing machine is normally provided with a spring structure which is disposed on the moving path of the thread to keep pressing against the thread. However, the above conventional structure has the following drawbacks: when the thread is pulled, the deformation of the spring is greatly increased, at this movement, the tension is too great, which will produce too much tension on the thread during a thread loosening operation, and thus the thread is likely to be broken. When the thread loosening operation is finished, the deformation of the spring is reduced, but the spring still keeps pressing against the thread with a certain amount of elastic force, so that an elastic tension still exists even when the thread doesn't need tension. As a result of this, when being pulled, the thread will also be broken due to an instant overlarge tension. In addition, although the spring is used to press against the thread to provide tension to it the tension of the thread is often either too great or too small, so it is difficult to perform the thread hooking operation assuredly during the thread loosening operation, thus causing the occurrence of skip stitch and needing improvement.
Further, when the thread is broken, since one end of the spring is provided with a hook which has poor sealing capability, the thread is likely to disengage from the hook. Further, it is also inconvenient to put it back into the spring once the thread is fallen out of the hook of the spring. Moreover, in order to control the change direction of the elastic force of the spring, the spring must be additionally provided with a framing structure to keep the deformation of the spring in a certain direction, thus inevitably increasing the structure complexity. In some other thread tensioning devices, the spring is totally replaced by a linkage cam, so that when the thread is pulled or loosened, due to the rotation of the cam, the thread can truly and selectively be kept in a tensioned state in a predetermined process and in a non-tensioned state in other processes. This structure is excessively complicated, so not only the production cost is higher, but the maintenance cost is also higher.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.